The education system rather than the teachers ________ for the overburden on the students. [ ]

A. are to answer  

B. is to answer

C. are answered  

D. is answered

烧伤后肺不张的主要原因有()

A．肺部感染

B．肺水肿

C．支气管内腔阻塞

D．低蛋白血症

E．休克